Deposition equipment

9 Squeegee durometer. Hardness level of the squeegee. Different colours will indicate the durometer number. The relation between colour and hardness may vary between vendors.

9 Emulsion. Photosensitive polymer on the bottom of the screen. It forms a gasket between the screen and substrate. A pattern is formed in the emulsion when the artwork is in contact and exposed to UV light. The unexposed pattern is washed away with deionised water.

9 Emulsion thickness. The thickness of the emulsion applied to the screen.

9 Mesh size. The number of openings in a screen. A 325-mesh screen has 325 openings per linear inch.

9 Flood printing. Printing done with the screen completely covered with paste. This is typical of high-volume operations.

9 Peel. The release of the screen from the printed area.

9 Planarity. Parallelism between the screen and substrate.

Figure III.3 shows the constituent parts of a screen printer. The screen fabric is attached to the screen frame, which is firmly held in a chase.

Figure III.3. Part of a screen printer

The screen is separated from the substrate by a snap-off gap of around 0.5 mm, which gives rise to the term off-contact printing process. The substrate is held in position by either a vacuum chuck or by a special jig, both of which are usually referred to as the substrate holder.

The platform upon which the substrate holder is mounted can be adjusted in the vertical plane at both ends in order to ensure that the substrate is parallel to the screen. The screen position can be finely adjusted for registration purposes. The printing medium is applied to the upper surface of the screen and the flexible rubber squeegee is traversed across the stencil. As the squeegee is passed across the screen, the mesh fabric is pressed into contact with substrate surface. The ink is forced through the open areas of the screen mesh. Directly behind the squeegee, the screen peels away from the substrate and leaves behind a deposit of ink, in the

required pattern, on the substrate surface. A screen printing machine is shown in figure III.4, where the essential parts can be clearly distinguished.

Squeegee Screen printer

control pad

Substrate holder connected to a vacuum

pump

Screen holder with screen

Figure III.4. Screen printer (Model: System Automation Inc. 810-12)

The screen mounting provides a means of holding the screen firmly so that it does not move during the printing cycle. It also provides a means for accurately positioning the screen for registration purposes. Typically, a positional tolerance accuracy of around 25 microns in the vertical and lateral directions is achievable. Although it is essential that the screen be held firmly in position, it is also necessary that it may be easily removed and replaced. If the screen is held in a chase, which is basically a screen carrier, it can be taken out and relocated accurately in the same position.

The function of the substrate holder is to register the substrate accurately below the screen and also to keep it firmly fixed in position during printing. The acceptable tolerances for the positioning and repeatability of the substrate holder are similar to those for the screen mounting. The two most common ways of holding the substrates are with a vacuum chuck or a recessed jig. In a vacuum chuck arrangement, the substrates are registered by butting two of their edges against two datum faces. The alternative is a recessed chuck, which is fitted with locating pins and a clamping edge. The depth of the recess is around 100 microns less than the thickness of a substrate.

The squeegee is essentially a flexible blade whose function is to transfer the paste through the screen and onto the substrate. During printing, the squeegee forces ink through the open areas

of the mesh and, by virtue of the surface tension between the film and substrate, the required pattern is transferred to the substrate as the screen and substrate separate. The squeegee's shape, material and pressure are all factors which dictate the life of the screen and the squeegee. Clearly, the squeegee must be resistant to the solvents and inks used in thick-film processing. Polyurethane and neoprene are common materials. Figure III.5 gives two examples of cross-sections of typical squeegees.

Figure III.5. Examples of different squeegee constructions

The angle of attack between the squeegee blade and the screen is usually around 45°. By using a rectangular cross section it is possible to prolong squeegee life as each of the four edges may be rotated periodically. The optimum durometer hardness of squeegees is in the range 50-70.

Squeegee down pressure can be adjusted, usually by spring loading. Other methods like pneumatic, dead weight and torsion bar loading can be found on some printing machines. It is essential to have accurate control over the squeegee pressure so that print thicknesses are accurate and repeatable.

The squeegee holder is sometimes mounted rigidly, but often it is a floating type:that is to say, it is pivoted so that it can align itself parallel to the screen. Although some printers print on both the forward and the return stroke, many operate with single direction printing as this gives improved registration. In order to return the ink to the starting position for the next print, a device called a flood blade is used. This is usually made of metal and is positioned perpendicular to, and slightly above, the screen. At the end of a print stroke, the squeegee is automatically lifted away from the screen and the action of the flood blade leaves a thin layer of ink over the surface of the screen ready for the next print stroke.

One of the most important aspects of thick-film inks is the viscosity, which must be accurately controlled if the quality of the film deposition is to be high. Figure III.6 shows how the viscosity of a typical thick-film varies at different stages during the printing cycle.

Figure III.6. Variation in the paste viscosity at different stages of the printing cycle Firstly, we must consider the general requirements for thick-film pastes in terms of the ink rheology. The characteristics required of the ink viscosity depend on the printing stage. When the ink is being forced through the screen, viscosity must be low. After printing, however, viscosity must be high because the film must retain its printed geometry and not run. For ideal (Newtonian) fluids, the viscosity is independent of the shear rate and only varies with temperature. However, for a thick-film paste the viscosity must change with the pressure applied. The classification for fluids of this type is termed pseudo-plastic.